Olefins have been traditionally produced from petroleum feedstocks by either catalytic or steam cracking. Unfortunately, the cost of petroleum cracking has steadily increased, making it important to find alternative feedstock sources for olefins.
Oxygenates, such as alcohols, are a promising alternative feedstock for making olefins. Alcohols may be derived from nonpetroleum sources, such as sugar. The fermentation of sugar produces ethanol. Alcohols also can be produced from synthesis gas. Synthesis gas can be produced from a number of organic materials, including but not limited to recycled plastics, municipal wastes, petroleum liquids, natural gas, carbonaceous materials including coal, and other organic material.
The prior art in the area of olefin generation from oxygenates, such as methanol and di-methyl ether, focuses on maximizing ethylene and propylene product yields as exemplified in U.S. Pat. Nos. 4,499,327, 4,677,243, 5,095,163, and 5,126,308. The total yield slate, typically includes light saturates with a molecular weight lower than ethylene, i.e. methane (CH.sub.4), hydrogen (H.sub.2), carbon monoxide (CO), carbon dioxide (CO.sub.2), and ethane (C.sub.2.sup.O), and heavier by-products with a molecular weight higher than propylene, i.e. C4's and C5's. In the prior art, little attention has been given to the overall optimization of the yield slate other than to minimize the C4's and heavier due to the fouling potential and relatively lower value of these by-products.
The production of the light saturates by-products has not been a problem to be addressed since these byproducts are "clean" compounds without any fouling potential and can be readily recovered for at least fuel value. Therefore, the prevailing focus in the prior art has been not to address minimizing the light saturates yields.
The disadvantage of this approach is that one must include costly separation facilities in the olefin production plant to first separate and then recover the methane and other light saturates from the desired ethylene and propylene products. Such recovery schemes typically include a cold box, a demethanizer, a deethanizer, and a ethylene/ethane splitter. Even though the various separations techniques are well known in the art, this equipment must generally operate at temperatures of -200.degree. C. (-328.degree. F.) and below, which require materials constructed of very expensive stainless steel alloys, as carbon steel piping becomes brittle and breaks when operating at temperatures below -100.degree. C. Heretofore, the prior art has not taught an effective way to minimize the methane and other light saturates yields to minimize the investment in such recovery facilities.
The production of methane from oxygenate feeds for a given catalyst can be reduced by lowering the reaction temperature. However, lowering the temperature also reduces catalyst activity and ethylene yield. The industry needs a method to produce olefins at high temperatures from oxygenates which achieves higher olefin yields with reduced light saturate yields.